Induction of the ferritin gene (ftnA) of Escherichia coli by Fe2+–Fur is mediated by reversal of H-NS silencing and is RyhB independent

Nandal, A., Huggins, C. C. O., Woodhall, M. R., McHugh, J., Rodríguez-Quiñones, F., Quail, M. A., Guest, J. R. and Andrews, S. C. ORCID: https://orcid.org/0000-0003-4295-2686 (2010) Induction of the ferritin gene (ftnA) of Escherichia coli by Fe2+–Fur is mediated by reversal of H-NS silencing and is RyhB independent. Molecular Microbiology, 75 (3). pp. 637-657. ISSN 0950-382X doi: 10.1111/j.1365-2958.2009.06977.x

Abstract/Summary

FtnA is the major iron-storage protein of Escherichia coli accounting for < or = 50% of total cellular iron. The FtnA gene (ftnA) is induced by iron in an Fe(2+)-Fur-dependent fashion. This effect is reportedly mediated by RyhB, the Fe(2+)-Fur-repressed, small, regulatory RNA. However, results presented here show that ftnA iron induction is independent of RyhB and instead involves direct interaction of Fe(2+)-Fur with an 'extended' Fur binding site (containing five tandem Fur boxes) located upstream (-83) of the ftnA promoter. In addition, H-NS acts as a direct repressor of ftnA transcription by binding at multiple sites (I-VI) within, and upstream of, the ftnA promoter. Fur directly competes with H-NS binding at upstream sites (II-IV) and consequently displaces H-NS from the ftnA promoter (sites V-VI) which in turn leads to derepression of ftnA transcription. It is proposed that H-NS binding within the ftnA promoter is facilitated by H-NS occupation of the upstream sites through H-NS oligomerization-induced DNA looping. Consequently, Fur displacement of H-NS from the upstream sites prevents cooperative H-NS binding at the downstream sites within the promoter, thus allowing access to RNA polymerase. This direct activation of ftnA transcription by Fe(2+)-Fur through H-NS antisilencing represents a new mechanism for iron-induced gene expression.